1. Field of the Invention
The present invention relates to a controller for a wire electric discharge machine, and more particularly to a control method capable of improving the machining speed and machining precision.
2. Description of the Prior Art
FIG. 18 is a view showing the outline of a conventional controller for wire electric discharge machine. A discharge pulse generator 1 applies voltage to a gap between a wire electrode 4 and a workpiece 5 in order to perform electric discharge machining, and is constituted by a DC power source, a circuit composed of a switching element such as a transistor, a charge/discharge circuit of a capacitor and the like.
A detection voltage generator 2 is a device for applying pulse voltage between the wire electrode 4 and the workpiece 5 in order to detect whether or not discharge can be performed at the gap between the wire electrode 4 and the workpiece 5, and is constituted by an active element such as a transistor, a circuit composed of a resistor, a capacitor and the like, a DC power source and the like.
A current-supply brush 3 is used to supply a wire electrode with current, and is connected to one terminal of the discharge pulse generator 1 and one terminal of the detection voltage generator 2 respectively. Also, the workpiece 5 is connected to the other terminal of the discharge pulse generator 1 and the other terminal of the detection voltage generator 2 respectively. Between the wire electrode 4 traveling and the workpiece 5, there is applied pulse voltage to be generated from the discharge pulse generator 1 and the detection voltage generator 2.
A discharge gap detection device 6 is connected to the workpiece 5 and the wire electrode 4, and judges on the basis of the transition of detection pulse voltage from the detection voltage generator 2 whether or not the discharging gap is in a dischargeable state. If dischargeable, the discharge gap detection device 6 generates a discharge pulse supply signal. Further, output from the discharge gap detection device 6 is processed by an equalizing circuit 22, and thereafter, is compared with output from a reference voltage setting device 23 to thereby obtain voltage deviation. The voltage deviation is inputted into a feed pulse arithmetic unit 24 in order to control feeding of the wire electrode. The output (pulse-like gap voltage of several μ seconds to several tens μ seconds) from the discharge gap detection device 6 is processed by the equalizing circuit 22 in order to match the output to the processing speed of the feed pulse arithmetic unit 24.
The feed pulse arithmetic unit 24 generates a pulse train, that controls the feed pulse space, on the basis of the voltage deviation to output to a feed pulse distribution device 12. The feed pulse distribution device 12 distributes this pulse train to driving pulses for X-axis and Y-axis in accordance with a machining program to output to a X-axis motor controller 10 and Y-axis motor controller 11 which drive a table with the workpiece 5 mounted thereon.
First, in order to detect whether or not discharge can be performed between the workpiece 5 and the wire electrode 4, detection pulse voltage is caused to be generated from the detection voltage generator 2 to apply to a gap between the workpiece 5 and the wire electrode 4. Then, a current is passed through between the workpiece 5 and the wire electrode 4. Then, when a voltage drop occurs between the workpiece 5 and the wire electrode 4, the discharge gap detection device 6 detects this voltage drop to judge it to be dischargeable, and transmits a discharge pulse supply signal to the discharge pulse generator 1.
As a result, the discharge pulse generator 1 generates a discharge pulse to supply the gap between the workpiece 5 and the wire electrode 4 with discharge pulse current. Thereafter, after the elapse of appropriate quiescent time during which the gap is cooled, the detection pulse will be applied to the gap again. The above described operation cycle will be repeatedly performed for electric discharge machining.
As described above, machining to remove one portion from the workpiece 5 is performed every time the discharge pulse occurs. More specifically, through the use of the detection pulse voltage, there is searched for a minute conductive passage of several tens μm or less to be formed in a gap between the wire electrode 4 and the workpiece 5, which are opposite to each other, and the discharge pulse current is flowed immediately for heating, transpiration or melting and splashing to thereby start discharging. An amount of removal per discharge pulse and machining performance differ dependent on magnitude of the discharge pulse current, characteristics such as heat of fusion, coefficient of thermal conductivity and viscosity during melting of materials of the wire electrode 4 and the workpiece 5, characteristics relating to cooling due to coolant (machining liquid) and sludge discharge, and the like.
Also, the next discharge subsequent to the generation of a certain discharge tends to concentratedly occur in the vicinity of a place where there exist a multiplicity of micro conductive passages through sludge which is mainly generated immediately after the previous discharge is terminated. For this reason, precise servo feed control and quiescent time control are requested so as to prevent discharges which occur one after another from being concentrated on one place.
FIG. 19 is a view showing a monitored waveform for machining voltage, machining current and machining speed when a square pillar of die steel shown in FIG. 13 has been cut out by a conventional method. At a corner where the direction of machining changes by right angle, idle feeding is performed by an amount corresponding to the gap in an instant. For this reason, the machining current decreases and the machining voltage becomes higher. Accordingly, the feed speed command becomes larger. After the direction is changed, the wire electrode and the workpiece approach to each other more than necessary to make the gap narrower, and it becomes difficult to discharge sludge smoothly. As a result, the sludge concentration becomes higher to cause discharge concentration, resulting in short-circuit or disconnection of the wire electrode.
For this reason, in the conventional control, it is necessary to provide servo feed (relative feed of the wire electrode to the workpiece) or discharge quiescent time within a fixed time or distance immediately after the corner, and further to separately add a process for confirming liquid pressure of the coolant in advance for evaluation. Moreover, precision correction to cope with change in plate thickness and shape of the corner of the workpiece is very complicated and difficult.
In the conventional feed control, as described above, since the detection is performed in terms of gap voltage, it lacks accuracy of feed, and disconnection easily occurs particularly in a state in which the wire electrode has been stretched tight, and therefore, it is not possible to comply with any desire to improve the machining speed. Also, particularly at a corner portion or the like in the shape of machining, disconnection easily occurs, and in order to prevent the disconnection, it is necessary to add a corner control process and the like such as decreasing the feed speed or the machining current.